FIG. 1 is a block diagram of protocol stacks for a control plane 100 for layer 2 (L2) user equipment (UE)-to-Network relaying. The L2 UE-to-Network Relaying feature studied in 3GPP Rel-15 allows a “relay UE” to serve as a link to the network for one or more “remote UEs”, according to the protocol stacks shown in FIG. 1.
So-called “group handover” is a natural optimization to reduce over the air signaling when a relay, serving one or more remote UEs, goes through handover. “Group handover” also helps to synchronize the handover procedures, so that race conditions do not occur between the relay handover and the remote handovers. One message (Group Handover Command) from the network to the relay UE results in separate handover commands being sent to all the remote UEs. The message may also include the handover command for the relay UE itself.
Group handover creates some complexities around the handling of the Packet Data Convergence Protocol (PDCP) layer. The handover command for each remote UE needs to be a complete Radio Resource Control (RRC) message, encrypted according to the security configuration of that remote UE. These already encrypted messages need to be delivered to the relay UE at substantially the same time as relay UE's own handover command, e.g., as Protocol Data Units (PDUs) inside an RRC message of the relay UE. Such an RRC message of the relay UE may carry the handover command for the relay UE as well as the PDUs for any remote UEs served by the relay UE. However, the basic relaying architecture cannot cause this to happen. A PDCP PDU processed for one remote UE would always be sent directly from the evolved Node B (eNB) to the relay, for forwarding to that remote UE. Therefore, it is desirable to provide a new system and method to enable the group handover.